They DO NOT check the pads for wear at all on the MOT. What they do check is only by visual inspection but if the discs have shown not to have much of a lip or scoring then they will pass.
This is true, they can do a visual inspection - normally the discs are visable, but if you have wheel trims / dust covers on the inside of the disc, it can be hard to see the pads. If they can see them, they pass the test, but appear low, they can advise that they are low, but this is only if they can see them, and they are not obligated to advise them.